The present invention relates to an HVAC unit such as a furnace or boiler which utilizes a burner or combustion chamber for burning fuel in order to provide heat. More particularly, the present invention relates to an ignition system or ignition control which lights or ignites a fuel such as natural gas, propane, or other combustible fuels.
HVAC units such as furnaces and boilers ignite and burn propane or natural gas or other fuel in order to provide heat. A furnace is generally comprised of a combustion chamber, an inducer/blower, a gas valve, an ignitor, and an ignition control system. The gas is burned in a combustion chamber or burner. Generally, an inducer/blower is coupled to the combustion chamber and provides combustion air to the combustion chamber. Combustion air is needed for efficient operation of the furnace. The combustion air generally increases the pressure within the combustion chamber.
Generally, furnaces are turned ON and OFF and otherwise monitored by a gas ignition control system or controller. The gas ignition control system is generally coupled to at least one pressure switch which is located in the combustion chamber. The pressure switch may be located before or after the combustion chamber as long as the pressure switch is in a position to sense or monitor pressure within the combustion chamber. The pressure switch is normally closed if the proper threshold pressure is reached in the combustion chamber. If the furnace is operated without proper pressure from the combustion air in the combustion chamber, a furnace malfunction may occur.
The gas ignition control system controls the operation of the inducer/blower, the gas valve, and the ignitor in order to ensure proper operation of the furnace.
The gas valves control the supply of gas to the combustion chamber. The gas valves, relays which control the gas valves, or both are electrically powered through the pressure switch. The pressure switch thus operates to automatically shut off the gas valves when the pressure switch is open. When the gas valves are shut off, the gas flame in the combustion chamber is extinguished. Alternatively, the ignition system may monitor the state of the pressure switch and shut off the gas valves when the pressure switch is open.
Gas in the furnace is ignited by the ignitor. Upon ignition, a "transient pressure wave" is produced. The "transient pressure wave" momentarily decreases the pressure in the combustion chamber and may cause the pressure switch to open although the inducer/blower is properly providing combustion air. The opening of the pressure switch in response to the "transient pressure wave" is undesirable because it prevents the proper operation of the furnace.
Heretofore, a delay circuit was used to prevent the opening of the pressure switch in response to the "transient pressure wave." When the pressure switch changes from an open state to a closed state, a delay circuit within the pressure switch closes an internal relay which shorts the pressure switch for a fixed period of time. However, the technique of using the delay circuit is undesirable because it is difficult to determine the proper length of time for the delay. Also, the delay circuit prevents the ignition control system from testing the operation of the pressure switch because the pressure switch is shorted internally. Further still, this technique is disadvantageous because the delay circuit is susceptible to failure which may extend the predetermined time. Yet another disadvantage is that the pressure switch with the internal relay and delay circuit is expensive.